This invention relates generally to micro-imaging and position sensing applications employing microlens arrays.
Scanning microlens arrays are useful for imaging and position-sensing applications such as wafer alignment tracking in maskless lithography systems. In a typical application such as a microlens lithography system (U.S. patent application Ser. No. 08/803,096), an object surface such as an alignment tracking pattern on a semiconductor wafer is illuminated in reflection mode through a proximate microlens array, and each microlens concentrates illumination onto a corresponding focus spot on the object surface. The reflected illumination transmits back through the microlenses and is projected onto a detector pixel array, and each pixel senses light from a particular corresponding microlens and focus spot. The detector signal is acquired as the surface is scanned across the focus spot array, and the signal data is analyzed to determine positional information and provide feedback to a position actuator mechanism (positioning servo).
The scanning microlens system makes it possible to acquire very high-resolution image data using comparatively low-resolution projection optics. But due to the projection optics"" low resolution, the detector-plane image field originating from a typical focus spot can have diffraction tails extending across multiple pixels, resulting in degraded image contrast and resolution. It is thus desirable to suppress or mitigate the effect of such diffraction tails.
One method that is commonly used to suppress diffraction tails is aperture apodization. An application of apodization in the context of microscopy is described in U.S. Pat. No. 5,859,424. Although this patent does not relate to microlens imaging systems, the disclosed apodization method and apodizer construction can be applied in an obvious manner to microlens microscopy and position sensing. A similar apodization method is discussed in U.S. patent application Ser. No. 08/803,096 in the context of microlens printing and lithography systems, and this method is equally applicable to microscopy and sensing. Although apodization can reduce the magnitude of the diffraction tails, it does not eliminate them entirely, and further mitigation of diffraction effects would be beneficial.
The effect of diffraction tails in microlens microscopy is strongly affected by the phase relationship between overlapping detector-plane image fields originating from different focal points. The effect is strongest when the fields are coherently superimposed and in phase. A similar situation can occur with microlens printing (U.S. patent application Ser. No. 08/803,096), wherein an exposure illumination field on a printing surface comprises a coherent, overlapping superposition of component fields originating from different source pixels in a spatial light modulator. Co-pending U.S. patent application Ser. No. 09/473,081 describes a method whereby the image source (i.e., the spatial light modulator) can be constructed to optimize the exposure field""s phase relationships and thereby mitigate the effect of diffraction tails. However, this method does not apply in an obvious manner to microlens microscopy and position sensing, because in this case the image source is an inspection sample or tracking pattern whose reflectance phase characteristics generally cannot be predetermined by design.
Although phase coherence effects are usually detrimental to microscopic imaging, some imaging techniques such as differential interference contrast microscopy exploit such effects to enhance imaging and position sensing capability. But such methods have not been applied with microlens imaging systems.
As indicated above, diffraction effects in microlens microscopy apparatus are strongly affected by the phase relationships between image fields originating from different focal points. The invention provides techniques for configuring the optical subsystem to mitigate, or exploit, the phase relationships. In this context, the optical subsystem refers to the microlens array, detector pixel array, and projection system.
Coherent phase interactions between the fields can be suppressed by inducing an optical phase shift in alternate microlenses so that the detector-plane optical fields originating from adjacent microlens focus spots have an induced relative phase shift of approximately xcfx80/2. (The phase shift can be induced, for example, by using a phase plate or by appropriate selection of the microlens thicknesses.) This design modification alone can significantly reduce coherence effects in the detector-acquired image if the object surface is flat and has a uniform reflectance phase distribution. Under more general conditions the same result can be achieved by superimposing two images that are acquired with the surface positionally shifted by one microlens unit between image acquisitions. The positional shift sign-inverts the image""s phase interaction component so that it cancels out in the superimposed image. (A microlens imaging system typically operates in a scanning mode, and the two images could be acquired by simply extending the scan range so that each object point is scanned by at least two adjacent microlenses.)
In the above-outlined embodiment of the invention, the phase-shifted microlens array preferably operates in conjunction with an apodization filter in the system projection aperture, which functions to minimize the magnitude of the diffraction tails and preferentially control their spatial distribution.
In an alternative embodiment, the apodization filter is replaced by a beam-splitting filter, which splits the energy from each microlens into two dominant diffraction lobes that illuminate two adjacent detector pixels. In this embodiment, each detector pixel senses coherently-superimposed radiation from two adjacent microlenses; and the invention functions to enhance, rather than suppress, the phase interactions. This embodiment provides phase contrast imaging capability for wafer alignment tracking.